抄録
The process of cortex formation in the apical meristems of normally elongating crown roots of rice plants was studied by observing cell arrangement and cell divisions. All longitudinal cell files in cortex of a root branched from the endodermal cell file (Fig.2). The branchings mostly occurred within 200 μm of the meristem tip (intersecting point of root axis with cap boundary). Consequently, the number of cortical cell layers decreased toward the meristem tip. Cortical cells dividing transversely were found as far as about twice the root diameter from the meristem tip (Fig. 7). The frequency of transverse cell divisions decreased gradually toward the meristem tip, from the 150 μm level in roots thicker than 600 μm in diameter and from the l00 μm level in thinner roots (Fig. 8). In thicker roots, no transverse division was seen in parts more distal than 50 μm. Periclinal cell divisions occurred only in endodermal cells near the meristem tip where transverse division frequency and layer number of cortical cells decreased (Fig.11). Most endodermal cells dividing periclinally are those from which cortical cell files are branched in a basal direction (Fig. 9). In thicker roots, periclinal divisions occurred in endodermal cells 50 μm-200 μm distant from the meristem tip; and in thinner roots, in those within 150 μm (Fig. 8). The transverse cell divisions in the most distal part of cortex with less number of cell layers are considered to decrease the number of cortical cell layers by sending this part in the basal direction. Thus, thicker roots with less frequency of transverse cell division in distal part of meristematic cortex tend to maintain their layer numbers in comparison with thinner roots. The periclinal cell divisions in endodermal cells act as if they compensate for the decrease in the number of cortical cell layers.
